London: Perth-raised Katherine Young, 30, joined in a civil union with her partner Isabel Todd, 25, at the British consulate in Sydney, three years ago this week.

“It was amazing and exciting,” Ms Young remembers. Their union gave her the right to go back to live in London with her loved one, despite the expiry of her working visa, and they have lived there happily ever after.

But it left a niggling sense that, despite winning the right to have their relationship recognised in law, they were still somehow excluded.

“A commitment is a commitment,” Ms Young says. “Civil union is similar but different (to marriage). It's like saying 'we will give you this feeling, and it's great, but at the same time we're not going to give you all the same rights as a heterosexual couple'

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“I don't believe we should be treated differently. We are still segregated and there's no need to be. It's 2013, it's time for us all to recognise that love is love.”

Tuesday London time will be the first test as to whether the UK parliament agrees.

Despite a furious last-minute Tory revolt, David Cameron's coalition government plans to introduce a bill putting gay marriage on the same legal footing as heterosexual marriage in England and Wales.

Same sex couples will be able to marry in civil and religious ceremonies.

The law will also allow civil partnerships to 'convert' into marriage – an option Ms Young hopes to take.

“We're very happy at the moment, but down the line we're thinking of having a family, and (marriage) is something we'd want to do first,” she says.

A new group called “Conservatives Grassroots” delivered a letter, signed by 20 senior party members, to Downing Street on Sunday pleading for the vote to be delayed until after the next election.

The letter said a “significant proportion” of party members felt it “extremely distasteful” that the law was being fast-tracked through parliament with limited time for debate.

They also said the law would damage their re-election prospects.

“To do so now, when the future of Britain's position within the European Union and the integrity of our own Union is in question and when the Party trails 10 per cent behind Labour in the latest polls, is a policy that a very significant number of Conservatives cannot support,” the letter said.

It cited a ComRes poll published on the weekend reporting that 20 per cent of Conservative voters would not support the party at the next election if same-sex marriage was legalised. Voting is not compulsory in the UK.

Conservatives Grassroots organiser and councillor Ben Harris, from right-wing think-tank the Bow Group, said polls varied widely on support for the bill, but it was clear there was not majority support within the Conservative party.

“It's being rushed through, there was no mandate for it in the 2010 Conservative manifesto or the coalition agreement, it was not mentioned in the Queen's Speech setting out the government's policies,” he said. “It is completely inappropriate to nip it in under the radar. Delaying until after the election would allow a proper debate for all the consequences, intended and unintended, to be fully debated.”

The party rests some of its re-election hopes on attracting disaffected Liberal Democrat voters, but the splinter group fears the gay marriage issue would prevent this. Mr Harris said gay marriage was also unpopular with ethnic minorities.

“In terms of detoxifying the party with the electorate, if you want to put it like that, and engaging with modern Britain, just under 66 per cent of ethnic minority voters oppose gay marriage according to a poll this month.”

Conservative MP David Burrowes, who has led opposition to the bill, claims that up to 200 of the 303 Tory MPs could vote against the bill. The Sunday Telegraph newspaper put the size of the revolt at around 180 MPs, including six whips and four members of the Cabinet.

Mr Burrowes says he has received hate mail and his children have been bullied because of his opposition to the bill.

Despite the split in the Conservatives, the bill is expected to pass the House of Commons with strong Labour and Liberal Democrat support.

Sam Dick, spokesman for lesbian, gay and bisexual charity Stonewall which has long campaigned for the law, says his organisation has been working hard to remind MPs of the support for the law in their electorates and in the general population, especially among young people.

A YouGov poll commissioned by Stonewall last year found strong backing for gay marriage.

“Polling shows seven out of 10 people support (gay marriage), and 80 per cent of younger British adults,” he said. “The government first committed to introducing equal marriage in 2010 so Stonewall have been pushing for a long time for a bill to actually be introduced to make it happen.”

Mr Dick said he was “cautiously optimistic” the bill would pass the House of Commons, and Mr Harris said he expected it to get through.

However Mr Dick said the House of Lords has “proved tricky” in the past on issues of social equality.

And Mr Harris predicted the Lords would “throw this back to parliament because it's poor policy”.

Civil partnerships were introduced in England in 2005, giving gay couples similar legal rights to marriage.

A survey last year found that a majority of voters agreed that a distinction between civil partnerships and marriage “worsens public attitudes towards gay people.”

In October 2011 Mr Cameron told his party conference he supported gay marriage “because I'm a Conservative.”

“To anyone who has reservations I say: yes, it's about equality, but it's also about something else: commitment. Conservatives believe in the ties that bind us, that society is stronger when we make vows to each other.”

A 13-week consultation last year received hundreds of thousands of responses. In December the minister for Women Maria Miller unveiled the government's response, saying it was “all about how society treats its citizens – we are all equal.”

The law will provide that no religious organisation can be forced to conduct gay marriages, but those that wish to, may.

The Church of England is specifically exempted from the law, and would not preside over gay marriage.